Bad News On Human Extinction

Disasters that destroy all but a thousand humans are more likely than disasters that destroy all but a hundred humans. So this news says human extinction is more likely than we thought:

Conservation biologists may be deluding themselves. An analysis of the minimum number of individuals needed for a species to survive in the long term has found that current conservation practices underestimate the risk of extinction by not fully allowing for the dangers posed by the loss of genetic diversity. If correct, it means the number of individuals in endangered species are being allowed to dwindle too far.

Lochran Traill at the University of Adelaide, Australia, and colleagues found that for thousands of species the minimum viable population size (MVP) – where a species has a 90 per cent chance of surviving the next 100 years – comes in at thousands rather than hundreds of individuals. Many biologists, Traill says, work with lower numbers and so allow unacceptably high extinction risks.

Disasters that destroy all but a thousand humans are more likely than disasters that destroy all but a hundred humans. So this news says human extinction is more likely than we thought:

How does this follow?

(Did you mean to say “Disasters that destroy all but a thousand humans are more likely than disasters that destroy all but a hundred thousand humans”? That doesn’t seem plausible, but at least your conclusion would clearly follow.)

Tyrrell McAllister

Let X < Y. Then disasters that leave up to X people alive are less likely than disasters that leave up to Y people alive (because all disasters that leave up to X people alive are also disasters that leave up to Y people alive).

In other words, disasters that leave up to Y people alive are more likely.

Suppose that, previously, we thought that we only needed more than X people alive to avoid extinction. Now we learn that, in fact, we need more than Y people. Before, we thought that we could only be wiped out by the rarer kind of disaster, the kind that leaves up to X people alive. Now we learn that we would be wiped out by the more common kind of disaster, the kind that leaves up to Y people alive.

I was confused too. I think it’s because Robin’s phrasing left out the “up to” which makes it sound like he’s talking about disasters that leave exactly 1000 or 100 people alive.

John Maxwell IV

The post is confusing because it looks like Robin means all but **exactly** a thousand humans.

AP

I’m baffled, I don’t see how this makes sense.

Jake

This raises the perhaps more interesting question of whether humans are an at-risk species.

Michael

It also raises for me the question of whether humanity’s extinction if to be strenuously avoided. I mean, I have a personal desire to not go extinct any time soon, but given h.sapiens‘ tendency to exterminate other lifeforms I wonder if the best result all round might be for us, as a species, to… go away.

Is this some kind of species-treason?

Someone from the other side

This is what I find fascinating about all these we have to colonize the solar system to not have all eggs in one basket arguments. Unless you spread *yourself* across the solar system, how exactly do you benefit?

Tyrrell McAllister

You benefit if you put any value on humans other than yourself, or on anything that is preserved if humanity if preserved.

The Traill paper seems to support a rough consensus in the MVP literature that for mammals the MVP is on the order of thousands. It is not that surprising – humans are large mammals with slow maturation, single births and long generation times.

I tried my hand at doing a bit of MVP for humans after discussing the issue with Robin. My estimates also ran in the thousands. The survival probability is roughly linear in initial population up to several thousand. To get 90% survival after 1000 years (more sensible than 100 for long-lived humans) with life tables corresponding to modern Sri Lanka I needed around 4500 people in the initial group. Adding occasional bad years (a 1/15 chance that a year had double mortality, and 1/150 that it had ten times mortality) reduced the survival probability to ~50% even with 5000 people. Running a low-mortality life table based on modern Sweden made things slightly better, ~60%.

My model did not take genetics into account, so it should be regarded as optimistic. I also had serious problems getting any survival with hunter-gatherer life tables, unless I tweaked them. Boosting average growth rate to 2% produced a MVP of a ~500 people – but that presupposes a very benign environment, not a likely occurence after a global disaster.

One interesting observation was that even a doomed population can linger for centuries. Conversely, small founder populations for isolated locations may be due to luck: we do not see any genetic traces of the unlucky ones, only the first population to by chance grow far enough to dominate the land.

If we are in the roughly linear survival probability range the chance of a population of size N surviving is kN. If we have N0 people divided into R refuges, each refuge has a chance kN0/R of succeeding. The probability that at least one refuge survives is 1-(1-kN0/R)^R. Given these curves a typical value for k is around 1/10000, I end up with a monotonously decreasing survival probability with R: it is smarter to have one big refuge than many small and vulnerable. However, a proper model would include the risk of local disasters which I think will improve the chances for distributed refuges. But in any case, this is a nice example of how even humans are vulnerable to habitat fragmentation.

The resources left behind will drastically affect survival, as will the efficacy of further farming effects.

(B) Assuming that most people are dead and that rice/wheat/potatoes are still viable staple crops, there would likely be huge surpluses for the forseeable future. We aren’t talking about hunter gatherers on marginal lands, we’re talking about a group of people with their choice of the best soils in the world, along with an unprecidented repopulation of wildlife (game). Human intelligence being what it is, that surplus could be intelligently used to support ridiculously high breeding rates. (This also gets into the alpha/beta discussions. Doesn’t MVP include basic assumptions on who’s breeding with who in the wild? There wouldn’t necessarily be any ‘wild’ in this scenario.)

(C) It’s possible to put 4000 people on the planet such that none of them would ever see another living soul again. On the other hand, 4000 survivors of the apocalypse could all be in a single place (say, Cheyenne Mountain). The import thing is that the population needs to be close enough to interbreed. Either way, unless there are piles of bodies left, disease basically goes away, which is another problem with using modern hunter/gatherer lifespans.

In short, wouldn’t genetics be all that mattered?

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